We’ve got a floor, but a room needs to have walls to really be able ot be called a “room” in the first place.
The walls are prefabs with a few different configurations. To easily modify a prefab without having to rebuild the whole thing, they can be “broken” by right-clicking and selecting Prefab > Unpack.
This reverts the object to a “normal” asset, allowing it to be freely modified, and then renamed and turned into a new prefab.
To easily replace one asset with another, parent it under the asset you’re replacing. The new asset will inherit the Transform data of its parent. Un-parent them and delete the original.